feat(database): add engine options

[jbrockopp]

This change adds a new EngineOpt{} type used to configure the database agnostic engine{} type introduced in #868.

This change also updates the database.New() function to accept a list of EngineOpt{}.

Additionally, I made some miscellaneous refactors to improve consistency across the database package:

• updated the Config{} type -> config{} since we no longer need it exported (a.k.a. public)
• updated the engine.Logger field -> engine.logger since we don't need it exported (a.k.a. public)
• renamed the engine.Database field -> engine.client since we don't need it exported (a.k.a. public)

[codecov]

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